Detecting registration marks with low energy electron beam

ABSTRACT

For electron beam wafer or mask processing, a registration mark is capacitively coupled to the top surface of an overlying resist layer on a substrate to form a voltage potential on the surface of the resist layer directly over the registration mark. The registration mark is directly connected to an electrical lead that produces an AC voltage on the registration mark, which is capacitively induced on the surface of the resist layer. Alternatively, the registration mark itself is capacitively coupled to a conductive plate placed on the bottom surface of the semiconductor substrate. An AC voltage is then applied to the conductive plate that induces a charge on the registration mark, which then capacitively induces a charge on the surface of the layer of resist. An electron beam scanning across the surface of the resist layer generates secondary electrons. The secondary electrons have a low energy and are affected by the voltage potential created at the surface of the resist layer. Thus, by detecting the secondary electron signal generated by the electron beam the voltage potential on the surface of the resist layer is detected in contrast with surrounding areas. Consequently, the registration mark is detected by an electron beam, such as a low energy electron beam produced for example by an electron beam microcolumn, that does not have sufficient energy to penetrate the resist layer.

This application is a divisional of Ser. No. 09/060,496 filed Apr. 15,1998.

FIELD OF THE INVENTION

The present invention relates to semiconductor processing and toregistration marks and in particular to detecting registration marks forlow energy electron beam lithography.

BACKGROUND

The use of registration or fiducial marks in semiconductor processing iswell known. Registration marks are used to align one pattern layer ofmetal, insulator, or semiconductor material on a substrate with anotherpattern layer to ensure that features of the successive layers bear thecorrect spatial relationship to one another. The features of theregistration marks are typically used to align the substrate with thelithographic writing tool being used, such as optical or direct electronbeam writing lithography. During the lithography process, theregistration mark is observed and used to properly align thelithographic pattern with the underlying layer. In optical lithographythe registration mark is typically observed with an optical scanner.Although this method may be used with direct electron beam writinglithography as well, where the registration mark is under a layer ofresist, the registration mark is conventionally "observed" by detectingthe back scattered electron signal generated when the electron beamcontacts the registration mark.

A conventional electron beam used in direct writing lithographytypically has a high energy level, in excess of 10 keV and up to 50-100keV. A high energy electron beam can penetrate a layer of resist with athickness of approximately 2000 Å to 2 μm and contact an underlyingregistration mark. As the electron beam penetrates the resist layer,back scattered electrons are produced. By detecting the contrast in theback scattered electron signal caused when the electron beam contactsthe registration mark under the layer of resist, the location of theregistration mark may be determined. The electron beam is then alignedaccordingly.

FIG. 1 is a side view of a semiconductor substrate 10 with aconventional registration mark 12 covered by a layer of resist 14. Aconventional high energy electron beam 16 is shown penetrating resist 14and contacting registration mark 12. Back scattered electrons, which areillustrated as arrows 18, are detected by electron detector 20. Aselectron beam 16 is scanned across resist layer 14, as illustrated byarrow 22, the contrast in the back scattered electron beam signaldetected by detector 20 will indicate the location of registration mark12. A conventional registration mark is typically a conductor of amaterial different from the substrate or a physical step or void in thesubstrate.

Thus, to detect underlying registration marks, conventional electronbeams must operate at an energy level that is sufficient to penetratethe layer of resist. Where an electron beam does not have sufficientenergy to penetrate the layer of resist to contact the registration markthere will be no contrast in the back scattered electrons to indicatethe location of the registration mark.

An example of a electron beam that may not have sufficient energy topenetrate a layer of resist is a miniature electron-beam microcolumn("microcolumn"). Microcolumns produce low energy electron beams,currently 1-2 keV, and thus may have difficulty detecting registrationmarks underlying the layer of resist that is greater than approximately1000 Å. Microcolumns are based on microfabricated electron "optical"components and field emission sources and may be used for direct writinglithography. Microcolumns are discussed in general in the publication"Electron-Beam Microcolumns for Lithography and Related Applications, "by T. H. P. Chang et al., Journal of Vacuum Science Technology Bulletin14(6), pp. 3774-81, November/December 1996, which is incorporated hereinby reference.

Thus, to detect a conventional registration mark, the electron beamsmust operate at an energy level that is sufficient to penetrate thelayer of resist. Where the resist layer has a thickness greater than thepenetration depth of the electron beam, one method to permit theelectron beam to contact the registration mark and generate acontrasting back scattered electron signal is to remove the resist inthe area directly over the registration mark. However, this extraprocessing step is undesirable because it is complex and time consuming.

SUMMARY

A registration mark that is under a layer of resist thicker than thepenetration depth of the viewing electron (charged particle) beam may bedetectable by applying an AC voltage to the registration mark tocapacitively induce a voltage potential on the surface of the layer ofresist. The registration mark is directly connected to an electricallead that applies an AC voltage across the registration mark. Acorresponding voltage potential is thereby capacitively induced on thesurface of the layer of resist. An electron beam scanning across thesurface of the layer of resist generates low energy secondary electronsthat are affected by the voltage potential on the surface of the layerof resist. By detecting the contrast in the secondary electron signal asthe electron beam is scanned across the surface of the layer of resist,the location of the registration mark may be precisely determined.Consequently, an electron beam with insufficient penetration depth tocontact the registration mark, such as a low energy electron beamgenerated by an electron beam microcolumn, may be used to detect theregistration mark.

The registration mark may also be capacitively coupled with a conductiveplate on the bottom surface of the semiconductor substrate. By applyingan AC voltage to the conductive plate, a charge is formed on theregistration mark, which then capacitively induces a charge on thesurface of the layer of resist directly over the registration mark.Thus, a registration mark under a layer of resist may be detected by thesecondary electron signal from an electron beam, where the registrationmark has no electrical connections.

BRIEF DESCRIPTION OF DRAWINGS

The above and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying figures, where:

FIG. 1 is a side view of a semiconductor substrate with a conventionalregistration mark covered by a layer of resist, where an electron beampenetrates the layer of resist to detect the registration mark usingback scattered electrons;

FIG. 2 is a side view of a semiconductor substrate with a registrationmark that is capacitively inducing a voltage potential on the surface ofthe layer of resist, where an electron beam is detecting theregistration mark using secondary electrons; and

FIG. 3 is a side view of a semiconductor substrate with a registrationmark that is capacitively coupled to a conductive plate on the bottom ofthe semiconductor substrate and the registration mark is capacitivelyinducing a voltage potential on the surface of the layer of resist,where an electron beam is detecting the registration mark usingsecondary electrons.

DETAILED DESCRIPTION

A registration mark may be capacitively coupled with the surface of anoverlying resist layer by applying an AC voltage to the registrationmark. The surface of the resist layer will have a voltage potentialdirectly above the registration mark that is in contrast with thesurrounding area. By detecting changes in the voltage potential withsecondary electron signals, an electron beam (or other charged particle)may be used to precisely locate a registration mark beneath the resistlayer without penetrating the resist layer.

FIG. 2 shows a side view of a semiconductor substrate 100 withregistration mark 102 under a resist layer 104, when registration mark102 may be detected from the secondary electron signals generated by anelectron beam 106 without the need to penetrate resist layer 104.Electron beam 106, which may be a low energy electron beam, is generatedby an electron beam source 107, such as an electron beam microcolumn.Electron beam 106 is scanned over the surface of resist layer 104, asindicated by arrow 105. Detector 110 is sensitive to changes in thesecondary electron signals that are caused by a potential voltagecontrast on the surface of resist layer 104. Although the presentdisclosure refers to use of low energy electron beams to detectregistration mark 102, it should be understood that the presentinvention may be used by any viewing electron beam that does not havesufficient energy to penetrate the thickness of resist layer 104.

Registration mark 102 is a conductive metal such as aluminum or platinumthat is sputter deposited as a layer. The layer of aluminum or platinumis then conventionally etched to form the desired registration mark 102.Of course, other conductive metals may be used, and the particularmethod used to deposit registration mark 102 may depend for example onthe type of metal used for registration mark 102 and at what point inthe semiconductor processing the registration mark 102 is beingdeposited. It should be understood that registration mark 102 may bedeposited on top of layers formed over the surface of substrate 100, andnot only on the surface of substrate 100 itself. Moreover, it should beunderstood that multiple registration marks may be appliedsimultaneously.

An electrical lead 108 of conductive metal formed on the substratesurface is directly connected to registration mark 102 and is relayed tothe edge of substrate 100 such that a voltage can be applied to lead108. Electrical lead 108 may be formed simultaneously with registrationmark 102 through the same process of deposition and etching or any otherappropriate manner. Electrical lead 108 may thus be made out of the sameconductive metal as registration mark 102. Although electrical lead 108is shown off the surface of substrate 100 in FIG. 2, this is for thesake of clarity. It should be understood that electrical lead 108 isactually deposited along the surface of substrate 100 or on top of alayer formed over the principal surface of substrate 100.

Resist layer 110 is then conventionally applied over registration mark102 and electrical lead 108, through deposition or spinning or otherappropriate manner. Although resist layer 110 is shown as only onelayer, it should be understood that resist layer 110 may be a number ofresist layers and/or intermediate layers, such as insulator orsemiconductor material, necessary for the processing of substrate 100.

By applying an AC voltage to registration mark 102 via electrical lead108, a capacitive charge is generated at the surface of resist layer 104directly over registration mark 102, as shown in FIG. 2 as a series of"-" signs. Thus, detection of the voltage potential at the surface ofresist layer 104 will indicate the location of registration mark 102. AnAC voltage of approximately 1-2 volts having an AC frequency between 2and 2k Hz may be used to generate an adequate voltage potential at thesurface of resist layer 104.

An electron beam incident on the surface of resist layer 104 generatestwo signals. A back scattered electron signal is generated as theelectron beam penetrates a resist layer and typically has the sameenergy as the electron beam that generated the back scattered electronsignal. A low energy secondary electron signal is also generated. Thesecondary electron signal is generated when the electron beam contactsthe surface of the resist layer and the secondary electron signal has anenergy level that is much lower than the incident electron beam thatgenerated it. For example, where a 1 keV electron beam generated by anelectron beam microcolumn is used, the secondary electron signal willhave an energy level of approximately 1-2 eV.

The trajectories of the secondary electrons will be influenced by thevoltage potential at the surface of resist layer 104. Thus, by detectingthe secondary electrons generated by electron beam 106 contacting thesurface of resist layer 104 as electron beam 106 is scanned across thesurface of resist layer 104, the location of an area on the surface witha voltage potential that differs from surrounding areas may bedetermined. Consequently, the location of registration mark 102underlying resist layer 104 may be determined with electron beam 106without penetrating resist layer 104.

Detector 110, which conventionally detects the secondary electronsignal, moves in conjunction with electron beam source 107 across thesurface of resist layer 104. Detector 110 is sensitive to changes in thesecondary electron signal indicating a contrast in the electrical chargeon the surface of resist layer 104.

Because electron beam 106 does not have to penetrate resist layer 104 inorder to detect the registration mark, a low energy electron beam may beused. Thus, an electron beam microcolumn or other source of low energyelectron beams may be used to generate the secondary electron signalused to detect registration mark 102. Moreover, during a direct electronbeam writing lithography process the same electron beam 106 that is usedto write on the surface of resist layer 104 may be used to generate thesecondary electron signal that is used to detect the location ofregistration mark 102. Of course, detection of registration mark 102 bya low energy electron beam is not limited to electron beam lithography.If desired an electron beam may detect registration mark 102 at othersteps in the processing of substrate 100. Thus, by providing an ACvoltage on registration mark 102, a voltage potential is generated onthe surface of resist layer 104 that advantageously permits a one to onecorrespondence between the viewing beam and the writing beam.

FIG. 3 shows semiconductor substrate 100 with registration mark 102underlying resist layer 104. The structure shown in FIG. 3 is similar tothe structure shown in FIG. 2, like designated elements being the same.However, in FIG. 3, registration mark 102 is capacitively coupled to thevoltage source by a conductive plate 212 instead of being directlyconnected to electrical lead 108 (shown in FIG. 2).

Conductive plate 212 may be placed on the bottom side of substrate 100.Conductive plate 212, which may be any metal such as aluminum that issuitable for use in a vacuum, covers the entire bottom surface ofsubstrate 100 or may cover an area that is slightly larger thanregistration mark 102 and that is directly under registration mark.

Taking advantage of the insulation properties of semiconductor substrate100, which is e.g. silicon, conductive plate 212 is capacitively coupledto registration mark 102. Thus, when conductive plate 212 receives an ACvoltage via electrical lead 208, a charge is generated on registrationmark 102, as shown by the "+" signs in FIG. 3, which in turn generates acharge on the surface of resist layer 104, as shown by the "-" signs inFIG. 3. Thus, as described above, the secondary electron signalgenerated from electron beam 106 can be used to detect registration mark102 under resist layer 104.

Because conductive plate 212 is capacitively coupled to registrationmark 102, electric lead 108 shown in FIG. 2 is not necessary. Thus, theprocessing of substrate 100 is simplified. Further, without the need toconnect all the registration marks on substrate 100 with electric leads,a greater area of substrate 100 may be used for devices.

Although the present invention has been described in considerable detailwith reference to certain versions thereof, other versions are possible.For example, the detection of registration marks by capacitive couplingthe registration mark to the surface of the resist layer is not limitedto a low energy electron beam. Secondary electron signals from a highenergy electron (or other charged particle) beam may also be used toprecisely locate a registration mark using an embodiment of the presentinvention. Further, the process steps to apply registration mark 102 andeither electric lead 108 or capacitive plate 212 is not limited to thosesteps described herein. Registration mark 102 may be applied to anylayer on substrate 100 at any desired step in the processing. Further,the present invention does not demand that registration mark 102 becovered specifically with resist layer 104, but that registration mark102 is simply covered with a layer that has insulator properties.Therefore, the spirit and scope of the appended claims should not belimited to the description of the versions depicted in the figures.

What is claimed is:
 1. A structure comprising:a semiconductor substrate having a principal surface; a conductive registration mark on said principal surface of said semiconductor substrate; at least one layer applied over said conductive registration mark, said at least one layer having a top surface; and a structure on said substrate which couples a voltage to said conductive registration mark, whereby said conductive registration mark is capacitively coupled to said top surface of said at least one layer.
 2. The structure of claim 1, further comprising at least one layer disposed between said semiconductor substrate and said conductive registration mark.
 3. The structure of claim 1, wherein said structure comprises an electrical lead directly connected to said conductive registration mark, said electrical lead providing said voltage to said conductive registration mark.
 4. The structure of claim 1, wherein said structure further comprises a conductive plate on an opposing surface of said semiconductor substrate, said conductive plate being capacitively coupled with said conductive registration mark.
 5. The structure of claim 1, wherein the location of said conductive registration mark on said semiconductor substrate is determined by the capacitive coupling between said conductive registration mark and said top surface of said at least one layer.
 6. The structure of claim 1, further comprising a plurality of conductive registration marks, wherein said structure on said substrate couples said voltage to said plurality of registration marks, whereby said plurality of said conductive registration marks are capacitively coupled to said top surface of said at least one layer. 